Agriculture Reference
In-Depth Information
acids such as eicosapentaenoic (EPA) and docosahexaenoic (DHA) (Støttrup 2003; Drillet
et al. 2006; Ajiboye et al. 2011) and a longer passage time through the fish gut (Pedersen
1984). Copepods can also be administered under different forms (nauplii, copepodites, adults)
adapted to different larvae sizes. Copepods with potential use in aquaculture belong to three
orders: Harpaticoids, such as Tisbe spp., are epibenthic copepods, with in many cases pelagic
nauplii; and Cyclopoids and Calanoids, such as Acartia sp., are pelagic copepods suitable for
feeding a wide range of species.
Fish larvae have a requirement of specific highly unsaturated fatty acids (HUFAs), such
as DHA and EPA, in their diet (Watanabe et al. 1983; Rainuzzo et al. 1997; Conceiçao et al.
2010). Cultured rotifers and Artemia do not contain these fatty acids in the amounts required for
the rearing of most fish larvae and need to be enriched using emulsions of marine oils, commer-
cial products or microalgae. Several microalgae species, including flagellates (e.g. Isochrysis ,
Tetraselmis and Rhodomonas ), diatoms (e.g. Skeletonema , Chaetoceros and Phaeodactylum )
or chlorococcalean (e.g. Chlorella spp.) are cultivated as food sources for zooplanktonic live
feed. Microalgae can be also delivered directly to the rearing tanks ('green water technique')
to improve water quality, bacterial control and growth/survival of fish larvae.
The bacterial colonization of the gastrointestinal (GI) tract of fish larvae starts after the
opening of the mouth, which occurs some hours/days after hatching, depending on the fish
species and culture conditions. The establishment of the GI microbiota is influenced by bacteria
present in eggs, water and live feed (Nicolas et al. 1989; Munro et al. 1994; Reitan et al. 1998;
Ringø and Birkbeck 1999; Olafsen 2001). The different stages of evolvement of gut microbiota
in the larvae can be summarized as follows:
(1) In the yolk-sac stage, the larvae represent a pristine environment practically devoid of
bacteria. After opening of the mouth, the pioneer colonizing species originate mainly from
the microbiota associated with the water, though the origin of bacteria associated with fish
larvae at this stage are multiple (fish eggs, inlow water, bacteria associated with tank walls
and other parts of the rearing system, and microalgae cultures). Marine fish larvae drink
seawater to osmoregulate, and accumulate bacteria from the water (Mangor-Jensen and
Adoff 1987). In fact, the numbers of bacteria in the gut are much higher than those in
the rearing water, indicating that there is a selective uptake of bacteria and/or bacterial
multiplication in the GI tract of the larvae (Reitan et al. 1998).
(2) From the onset of the exogenous feeding, the numbers of bacteria increase exponentially
and the diversity of the microbiota decreases. At this stage, opportunistic species, mainly
derived from the ingested live feed organisms, increase in number by several log scales
(Bergh et al. 1994; Skjermo and Vadstein 1993).
Bacteria colonizing the intestinal mucosa in first-feeding larvae can provide protection
against potential pathogens (e.g. competition for attachment sites or nutrients, production of
inhibitory compounds, etc.) and interact with the host (e.g. enhancing the immune response,
improving GI morphology and digestive function) (Hansen and Olafsen 1999; Bergh 1995;
Gómez and Balcázar 2008). However, live feeds are also potential carriers of pathogenic or
opportunistic bacteria (Verdonck et al. 1997), which can increase larval mortality.
Basic knowledge of the bacterial communities associated with live feeds and the develop-
ment of methods to control bacterial microbiota in live feed production are crucial to enhance
larval survival. The development of strategies other than the use of antibiotics and disinfectants,
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